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Loss of H2 and CO from Protonated Aldehydes in Electrospray Ionization Mass Spectrometry



Pedatsur Neta, Yamil Simon, Yuxue Liang, Stephen Stein


Electrospray ionization mass spectrometry of 3-formylchromones and 3-formylcoumarins shows that collision-induced dissociation of the protonated ions results in elimination of H¬¬2 as a major fragmentation route. This loss of H2 yields an unstable carbonyl cation, which is susceptible to rapid reaction with water to form a protonated carboxylic acid. This process can take place in the electrospray source and in the collision cell. Consequently, the fragment ions observed in the MS/MS spectrum include those derived from the aldehyde and from the corresponding carboxylic acid. Elimination of CO from the original protonated ions is observed as a minor route in these aldehydes, while other aldehydes exhibit loss of CO and little loss of H2. Studies reveal that aldehydes which protonate predominantly on their carbonyl group or contain a remote basic site for stable protonation, such as a pyridine ring, undergo loss of CO. But aldehydes which contain a vicinal group that forms a hydrogen bridge with a protonated formyl group undergo significant loss of H2, which leads effectively to oxidation and to more complex MS/MS spectra. In the chromone and coumarin derivatives, this complication arises when the formyl group is located on a ring position adjacent to the oxo group. DFT calculations show that elimination of H2 from the protonated aldehyde leaves behind a relatively stable acylium ion, better described as a ketene cation. Hydration of this cation can lead to more complex MS/MS spectra, which may distort MRM results and lead to erroneous identification in library search results.
Rapid Communications in Mass Spectrometry


Aldehydes, collision-induced dissociation, fragmentation, tandem mass spectrometry, formylchromones, formylcoumarins, H2 elimination


Neta, P. , Simon, Y. , Liang, Y. and Stein, S. (2014), Loss of H2 and CO from Protonated Aldehydes in Electrospray Ionization Mass Spectrometry, Rapid Communications in Mass Spectrometry, [online], (Accessed May 26, 2024)


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Created September 14, 2014, Updated October 12, 2021